The present circuit finds one use in a situation where signals are being converted from lightwave signals being carried over fiber optic cables to electronic signals for transfer over metallic transmission media. Thus, the signal amplitude does not vary significantly for any particular physical location. However, each particular physical location may have a greatly varied gain requirement from other applications of the amplifier. Thus, the typical prior art approach was to install an amplifier and adjustable optical attenuator and adjust the attenuator while using test equipment to measure the output signal at a given load point. The present invention eliminates the need for the test equipment and the need for providing the adjustment. Further, since the device has such a wide dynamic range, the present invention eliminates the need for further adjustment as aging of the components occurs.
Prior art attempts to advantageously control the dynamic range of an AGC amplifier, for a use such as outlined above, have proceeded along the lines of placing a variable impedance such as a controlled FET in parallel with the feedback resistor of the AGC amplifier. A problem with this approach is that for each variable impedance setting, a different transfer characteristic compensating set of impedance values should be provided. Typically, some compromise value is used which provides less than desirable transfer characteristic results for most values of input signal amplitude.
The present invention is operating in one embodiment at a frequency in the neighborhood of 100 MHz. When amplifiers operate at these frequencies, they are very sensitive to stray capacitance. If the switch used to connect the additional one or more feedback networks in parallel with the main feedback network is connected to the output of the amplifier as opposed to the input, or is not in and of itself a very low capacitance switch, the circuit will not work. People in the industry have been attempting to accomplish the effect of the present invention by various similar embodiments for a number of years, and the present embodiment is the first to succeed.
Another problem in the prior art approaches is that if a signal sensor is used to switch the amplifier between gain adjustments and it is based on a single input signal amplitude, a switching back and forth in a communications environment can cause considerable data disruption. Thus, it is essential that there be some type of hysteresis between the input signal values at which switching of the feedback circuit, in and out of operation, occurs.
It is also essential to reliability and accuracy of signal transmission that the switching be accomplished in some minimal time such as 300 ms.
It is thus an object of the present invention to overcome various deficiencies of prior art designs and to conform to customer requirements in electrical specifications of a communications type trans impedance amplifier.